A systematic sample from a frame of units is one drawn by listing the units and selecting every kth element of the list. For example, if there are N units in the frame, and we want a sample of size N/10, we would take every tenth unit: the first unit, the eleventh unit, the 21st unit, etc. Systematic samples are not random samples, but they often behave essentially as if they were random, if the order in which the units appears in the list is haphazard. Systematic samples are a special case of cluster samples.